Conventionally, steel materials are used as materials for vehicle bodies and the like in transport machinery such as automobiles. Steel materials used as components of transport machinery and the like are exposed to rain water and the like when used. Therefore, their surfaces are treated with antirust galvanizing. This provides waterproof effect by an oxide film formed on the galvanized surface, and also allows zinc to be preferentially corroded over iron (sacrificial protection) even when there are flaws or pin holes on the surface of the steel material.
Recently, hybrid automobiles, electric vehicles and the like have been rapidly studied and developed from the perspective of environmental protection, and weight reduction of vehicle bodies and other components of these automobiles has been required to improve fuel efficiency and for other purposes. Moreover, in order to achieve weight reduction of vehicle bodies and other components, constituting a portion of a steel material used as a material of such components by aluminum or an aluminum alloy material (hereinafter, aluminum materials and aluminum alloy materials are collectively referred to as aluminum alloy materials) has been examined.
Therefore, to produce vehicle bodies and other components, a steel material and an aluminum alloy material need to be welded by different-material bonding. Examples of techniques of different-material bonding of a steel material and an aluminum alloy material include a method of bonding base materials by MIG welding or laser arc welding while feeding a flux-cored wire (patent document 1).
MIG (Metal Inert gas) welding includes feeding an inert gas such as argon or helium as a shielding gas to the vicinity of a site to be bonded, generating an arc between the welding wire and the site, and fuse-bonding the steel material and the aluminum alloy material. This MIG welding is characterized in that the welding operation is performed in a state that the atmospheric air is displaced by the shielding gas, and therefore welding proceeds without any influence of oxygen in the air. In laser welding, in contrast, a joint portion and a welding wire which is being fed to the joint portion are irradiated with laser light to thermally fuse the welding wire and joint portion by the laser light.    [Patent document 1] Japanese Unexamined Patent Publication No. 2003-211270    [Patent document 2] Japanese Unexamined Patent Publication No. 2008-68290